Fuel cells are electrochemical devices that convert directly the chemical energy of the reactant and oxidant into low voltage direct current electricity. The proton exchange membrane fuel cell (PEMFC) is the most promising of all fuel cell systems, its features making it an attractive candidate for stationary and automotive applications. The basic single cell PEMFC consists of an anode and a cathode bipolar plate, each provided with a network of channels, hereafter flow-field, formed in the electrically conductive material mass. The flow field is usually formed in the central portion of the bipolar plate, while the peripheral portion of the bipolar plate that surrounds the flow field is generally used to support sealing gaskets. The planar sides of the bipolar plates containing the flow fields are in contact with the gas diffusers, which generally consist of a porous electrically conductive material that may be carbon fiber paper or carbon fiber cloth, most of the times impregnated with mixtures of black carbon and PTFE. The gas diffusers delimit the forth side of the channels where the gaseous reactants flow. The solid matrix of the gas diffusers provides path for the electrons to be transported between the bipolar plates and the catalyst coated membrane (CCM). The CCM generally consists of a solid, ion conducting, polymer membrane impermeable to gases and two catalyst layers bounded on each side of the membrane. The catalyst layers are mixtures of electrically conductive carbon grains, platinum supported on carbon as catalyst, ionomer soaked with water as solid electrolyte and possibly PTFE as binder and hydrophobizing agent.
The sandwich defined by these elements is enclosed between two distribution-compression plates that provide inlet and outlet manifolds for the reactants and coolant, as well as an adequate compression level for the stack. The compression level of the stack is adjusted by threaded tie rod-nut assemblies, generally placed along the peripheral part of the fuel cell stack. An adequate stack compression level is required in order to insure a low interface resistance between the fuel cell components, as well as an optimum sealing compression for the gaskets.
In a PEM fuel cell, hydrogen rich fuel is fed through the anode flow-field, while atmospheric air, or substantially pure oxygen is fed through the cathode flow-field. The gaseous reactants are transported by convection and diffusion throughout the channels and, depending on the technical solution of the flow fields—solely by diffusion (continuous-channel flow fields) or both by convection and diffusion (discontinuous, or interdigitated flow fields) throughout the gas diffusers, towards the CCM. The hydrogen molecules break into protons and electrons in the anode catalyst layer. Electricity is generated when the electrons pass through an outer electrical circuit and the positive ions pass through the solid polymer membrane towards the cathode catalyst layer and react with the oxygen to produce water.
In order to improve the ion transport across the CCM, the gaseous reactants are generally humidified upstream of the fuel cell inlet manifolds.
The planar area defined by the intersection of the projections of both flow fields, gas diffusers and catalyst layers areas on the membrane, is generally known as the active area of the fuel cell. This area is electrochemically active. The effective active area of a fuel cell may actually be smaller during fuel cell operation if the gas flow channels become partially filled with liquid water.
Prior art flow-fields generally consist of a plurality of parallel, continuous channels extending from a supply passage at one end, to an exhaust passage at the other end (continuous-channel flow fields). The continuous channels may follow a serpentine trajectory (see e.g. U.S. Pat. Nos. 4,988,583 and 5,108,849) or other trajectories (see e.g. U.S. Pat. Nos. 5,686,199; 5,773,160; 6,586,128B1; 2003/0077501; U.S. Pat. No. 6,541,145B2). A common characteristic of all the continuous-channel flow fields is that the reactants are transported only by diffusion in the gas diffuser, excepting the thin boundary layer at the interface between the channels and the gas diffuser, where convection is still present (Gurau et al.: “Two-Dimensional Model for Proton Exchange Membrane Fuel Cells” AIChE, Vol. 44, No. 11, pp 2410-2422, 1998). As a consequence, liquid water may be removed from the gas diffusers only due to capillary forces, which cannot be controlled during the fuel cell operation; this results in a low effective porosity of the gas diffuser, which affects negatively the reactant diffusive transport. Another common characteristic of the continuous-channel flow fields is the decrease of the reactant concentration along the channels from the supply passage to the exhaust passage, which determines a non-uniform reaction rate over the active area, and therefore a non-uniform utilization of the fuel cell.
An alternative to the continuous-channel flow fields are the discontinuous, or interdigitated flow fields (U.S. Pat. Nos. 5,300,370; 5,641,586; 6,551,736B1) consisting of a plurality of parallel high-pressure channels extending from a supply passage to a dead or closed end, alternating between parallel low-pressure channels extended from a dead or closed end to an exhaust passage. The major characteristic of the discontinuous, or interdigitated flow fields is that the gaseous reactants are transported from the high-pressure channels to the low-pressure channels throughout the gas diffusers, mainly by convection. This results in a two-phase drag of the liquid water out of the gas diffuser and in a better transport of the reactant-reach gaseous mixtures towards the catalyst layer, both having a direct positive influence on the fuel cell performance. The interdigitated flow fields are also characterized by a more uniform current density, due to an even distribution of the reactants along the catalyst layer.
U.S. Pat. No. 6,551,736,B1 discloses an interdigitated flow field consisting of only two channels in a spiral configuration, with the low-pressure channel placed at a lower radius than the high-pressure channel. In this configuration, the shorter low-pressure channel will be adjacent both sides to the same, longer high-pressure channel this resulting in an improved liquid water removal from the flow field.
The advantages of the interdigitated flow fields over the continuous-channel flow fields are hindered by the impossibility to remove the liquid water accumulated in time at the high-pressure channel dead or closed ends during accidental over-humidification of the reactants, or as a consequence of vapor condensation in the flow field during non-operating time periods.
The pressure differential Δp necessary to displace a fluid of dynamic viscosity μ with velocity u along a distance d through a porous gas diffuser material of porosity ε and a permeability to that fluid k is:
      Δ    ⁢                  ⁢    p    =            d      ɛ        ⁢          μ      k        ⁢    u  Since the value of the ratio
  μ  kfor liquid water is many orders of magnitude higher than that for gasses, the pressure differential necessary to force the liquid water accumulated in the high-pressure channels through the gas diffuser would be many orders of magnitude higher than the pressure differentials affordable during fuel cell operations. The liquid water accumulated in time in the high-pressure channels determines a decrease of the fuel cell effective active area.
The advantages of the interdigitated flow fields over the open-channel flow fields are not noticeable at low current densities, when the performance of the fuel cell is dictated by polarization limitations instead of mass transfer limitations. Since the interdigitated flow fields require higher pressures at the fuel cell inlet then the open-channel flow fields, the use of interdigitated flow fields for fuel cells operating at low current densities is not justified. Of great interest would be a flow field able to operate as an open-channel flow field at low current densities and as an interdigitated flow field at higher current densities.